Relevant prior art as claimed is based on an apparatus according to U.S. Pat. No. 4,597,093, which issued on June 24, 1986. There, a proportional counting tube 97 is shown. An X-ray generator 33 sends and X-ray beam to a target 92, which is excited to produce X-ray fluorescence and the excited radiation is radiated to the counting tube 97. A closing device 41 is drawn in the OPEN position. In order that no radiation can emerge from the opening 22 when the latter is open upward, the plug 41 can be turned by means of an operating device (not shown) in such a way that its bore hole 42 is horizontal. It is then in the CLOSED position.
The plug 41 can be imagined as the rotatable part of a beer faucet. It is clear, however, that the closing device does not have to be rotationally operable. Rather, it may also be translationally operable.
It is clear that such a measuring instrument--like any other--has a drift, induced by effects such as heat, age, power supply, etc. In order that an intensity measurement can be carried out with a certain characteristic X-radiation, it is necessary that the amplification of the "proportional counting tube--charge amplifier--main amplifier--multi--channel analyzer" system operates stably. A stabilization of this measuring system is achieved in the prior art in that a radiation with a defined spectrum is irradiated onto the counting tube and the entire measuring arrangement is stabilized such that the line of this spectrum always lies in a certain position. This is not possible during measuring. Usually, at certain periodic intervals, instead of an object to be measured (target), a reference object is brought into the primary ray. It is either introduced manually or swung in automatically and then the counting tube is stabilized. The disadvantage of this process is that the system is operated unstabilized between the individual stabilization phases.
It is typical of layer thickness measurement in the prior art that a break always occurs after short measuring time intervals (between 1 and 20 seconds), which break may extend to several minutes. In these breaks, a different object to be measured is positioned. For radiation protection safety reasons, the primary radiation then has to be screened off in the measuring room. Or there are small measuring tasks to be performed and the instrument is not operated, but remains switched on. In this case too, X-ray protection regulations specify that the radiation must not be emitted into the accessible measuring room. In order to meet the requirement for radiation protection, after completion of a measurement, the outlet opening of the primary X-ray is screened off by a closing device.